DescriptionAt the beginning of the 1960s, cotton was Mexico’s most valuable crop and its biggest export, and the Mexicali Valley in Baja California was Mexico’s most productive cotton-growing region as well as a centerpiece of its agrarian reform. So when the waters of the Colorado River, whose water provided arid Mexicali its irrigation water, suddenly became highly salty in the autumn of 1961 as a result of agricultural drainage upstream in Arizona, locals and government officials alike reacted with alarm, sparking what became a twelve year long diplomatic dispute between Mexico and the United States and a political crisis within Mexico itself. The story of the Colorado River salinity crisis provides new insights into one of the most persistent questions of Mexican history: how the country’s single-party regime evolved and endured for so long, and how its features shaped contemporary Mexico. While scholars have detailed how corruption, co-optation, and culture supported the regime, few have examined how environmental change shaped and reflected the state’s rule, even though its claim to revolutionary credibility rested in large part on its promise to nationalize water and subsoil rights and to remake agrarian society. Combining environmental and political history, this dissertation argues that the changing ecology of the Lower Colorado River undergirded the evolution of Mexico’s authoritarian political system during the Cold War of the 1960s and early 1970s. The problem of salinity gave teeth to a nationwide leftist challenge to the ruling party’s grip on power, inspired in part by the Cuban Revolution, and which took up salinity as a rallying cause. With the resolution of the problem mired in ecological complexity and diplomatic impasse, the Mexican government sought a solution through foreign policy. Tacking leftwards and embracing relations with revolutionary Cuba as a sop to domestic leftists, Mexican officials warned their U.S. counterparts that the salinity problem was catalyzing Communist agitation within Mexico. To increase the pressure, the government began encouraging and facilitating anti-U.S. protests in Mexicali, while ruthlessly repressing those that targeted the regime itself. The strategy won concessions from the United States in 1965 and 1973 agreements, and helped to defuse the leftist challenge by the mid 1960s. At the same time, the dissertation argues that the nature and exigencies of Mexican authoritarianism were the driving force in the ecological transformation of the Colorado River Delta from the late 1960s onwards. The political utility of the salinity issue to induce loyalty to the ruling party at a time of increasing opposition caused the regime to double down on its pursuit of irrigated cotton agriculture in Mexicali, against the evidence from its own scientists of cotton’s unsustainability, not just from salinity but from other ecological and hydrological problems. Concessions from the United States helped to fund a massive irrigation infrastructure project in the 1970s, designed to restore both the prosperity of cotton and political stability, which eventually achieved neither but inadvertently nudged forward a trend towards bi-national cooperation and ecological restoration along the river. Based on a wide range of archival sources, the dissertation contributes to our understanding of Cold War Mexico’s domestic politics and foreign policy, and shows how techno-ecological change and political authoritarianism both strengthened and undermined each other, a finding with broad implications beyond Mexico.